Actually, as I stated before the scrap piece of sheeting that you use is 1/16", and the rail is 3/16" (just never stated the rail size).
The following is straight from the manual:
Use a scrap piece of 1/16" balsa to support the rail 1/16" above the fuselage side and secure it with pins, from the outside of the fuselage.
If you wanted to you could put a piece of scrap all the way across the fuse and set the rails on top.
The point here is that it doesn't matter what size of scrap sheeting you use since the only purpose that is serves is to elevate the rails so that they can be aligned properly and glued to the hatch. Once this is done they are removed and thrown back into the scrap pile.